How many people understand general relativity?

[KimB]

If you think angular momentum is intuitive, then let me ask you this question:

Let's say I take a bicycle wheel that's not attached to a bike, but has a pair of pegs attached for handles. On one peg is an eye hole with a rope attached. Now, let's say I set the wheel spinning. For the sake of the example, I'm holding onto the pegs, one in each hand, with the eye hole in my right hand. I spin the wheel so that the top is coming towards me. I then let go with my left hand, and hold the wheel by the rope from the right eye hole.

What happens?

 

[Davros]

you hit yourself in the head

 

[KimB]

you hit yourself in the head

Cute But no, it's easy. And pretty neat, actually.

 

[Richard]

As for being able to visualize dimensions >3. Well, that too is a little weird, and can depend what you mean. There have been famous mathematicians/topologists in the past who have uncanny abilities to 'see' 3 dimensional embedded cross sections of higher dimensional surfaces.

A bit tongue in cheek but I'm always visualising the world in four dimensions.

 

[Bambers]

Woo its a gyroscope. Classic example unfortunately, however given the fact you didn't specify the fact that the wheel was freely rotateable it would likely try to wind itself up the rope, the added upwards force on the right peg being enough to tilt the wheel. and once it tilts the rope will no longer have something to wind around and it'll drop and start bouncing all over the place (though with an underlying clockwise rotation when viewed from above as initially as gravity - and the winding - attempted to tilt the wheel the right peg would also have felt a force towards you) actually likely ending with Davros's conclusion

but I'm not sure what this is going to prove... I did do a Physics degree and I'm sure many of us have owned a gyroscope (apparently I've binned mine )

 

[Davros]

actually likely ending with Davros's conclusion

See i keep telling you people im a genius

 

[KimB]

Well, since you couldn't spin it up in the first place if it wasn't freely-rotateable (since you're holding onto one of the pegs while spinning the wheel), I guess it didn't occur to me to specify that.

 

[Arwin]

I have always intuitively believed that anyone who took general relativity to mean that time travel (particularly into the past) is possible was wrong. (I always wondered whether Einstein himself believed that, and didn't think so, and I think I came across a note that he wrote himself saying that time travel wasn't possible).

I did a presentation once where I used linguistic aspects of the word time to show that it didn't exist as a physical entity, that it was as abstract and utilitarian as currency and money is to relative value, as time is to relative movement with the conclusion that time travel was only possible when travelling forwards, because you can freeze an individual and let the rest of the world move on, then unfreeze the individual. Didn't get much of a response though from my bewildered audience (language majors) and my 'teacher' (not even officially qualified to teach at university level) thought the exact same of me, ironically.

I could be horribly wrong, of course.

The only strong 'epiphany' I've ever had was on structures (at any level) challenging each other. It was nothing special and a blend of all sorts of stuff, but a lot of things (neuro-connections) came together at once and it was quite a rush at the time. Ditto on having an intuitive understanding of how the conscience works. Me and a friend actually worked that out a fair bit at the time and today I still believe that's the way it works basically and at any rate. Of course by now the more interesting question is what effect a scientific explanation of (self)consience will have on society. Probably not nearly as big as geology and evolution had in the late 19th century, but I'm sure there will be some effect, as various cultures, religions and individuals will have to adjust (or simply ignore) to this part of life that has so far yet remained an 'official' mystery.

Though I have to say, gaining an understanding the basic functionality of neurotransmitters (taking dopamine and serotonine as basic examples) on human behaviour for me was an enormous breakthrough, but that doesn't seem to have reached most of the rest of the world yet.

 

[KimB]

Arwin, how the word time is used in general parlance is only peripherally related to how it's used in physics. In physics, time is a mathematical quantity. You cannot use linguistic arguments to say anything at all about a mathematical quantity.

 

[Arwin]

Arwin, how the word time is used in general parlance is only peripherally related to how it's used in physics. In physics, time is a mathematical quantity. You cannot use linguistic arguments to say anything at all about a mathematical quantity.

Except maybe I explained that the word time always relates to a mathematical quantity even in every day use, but we tend to sometimes forget that?

 

[KimB]

Except maybe I explained that the word time always relates to a mathematical quantity even in every day use, but we tend to sometimes forget that?

As long as you're not trying to make any statements about how time is used within a physical theory, and are only talking about the general populace's perception of time, I don't think there's much to argue about here. But from what little you've said, it sounds like you're attempting to make a rather wrong statement about the nature of the arrow of time, as it is used in physics.

 

[Arwin]

As long as you're not trying to make any statements about how time is used within a physical theory, and are only talking about the general populace's perception of time, I don't think there's much to argue about here. But from what little you've said, it sounds like you're attempting to make a rather wrong statement about the nature of the arrow of time, as it is used in physics.

Could you explain this further? I mean, in the sense of physical time vs mathematical time, you could say that if you take language and meaning Derrida style, then physical time is the signifiée, and mathematical time (the concept of time that you attribute to the general populace's perception of time) is the signifiant.

I think you'll find that in the end there's not so big a difference. What I did in the presentation is show people what we use and have used to measure time in the course of history. This shows that we have typically used a moving thing to measure the movement of another moving thing (in order to plan or predict, etc.). This gives a hint that time does not physically exist itself.

I don't see how this would be at odds with either mathematical time or physical time as used in physics. On the other hand, the concept of mathematical time taken too literally (as I think has been done a lot) by some physicists is possibly a strong contribution to the increased distance between signifiant and signifiée in the minds of 'casuals' in the first place.

Or am I missing something here?

 

[KimB]

The mathematical time is a description of real time. To the best of our understanding, the mathematical description of time in General Relativity is a very accurate description of the phenomenon.

All that your statements show is that we define time through observation. That's the way it should be done, after all: we wouldn't want to go off drawing conclusions about the nature of the real world without testing those conclusions through experimentation.

Remember that time isn't just about the fact that things move, but about the specific relationships between different motions. The fact that we can predict the motions of physical systems to an extreme degree of accuracy using time attests to the fact that time, when combined with the other aspects of these theories, is an accurate description of reality.

 

[Bambers]

Well, since you couldn't spin it up in the first place if it wasn't freely-rotateable (since you're holding onto one of the pegs while spinning the wheel), I guess it didn't occur to me to specify that.

I was initially working off the idea that the wheel was being spun by turning the pegs. Once I realised it wasn't clothes pegs being talked about anyway...

Regardless, by the time I had figured out the question I'd already recalled a fair bit about such things from my degree so I'm not sure whether my answer was based on that or more intuitive senses. However I don't find the actions of gyroscopes 'illogical'.

 

[KimB]

Heh, no, I meant pegs as in the devices that riders attach to their wheels so that they can do certain tricks, such as this:

However I don't find the actions of gyroscopes 'illogical'.

Well, it's a bit more interesting than a simple gyroscope, though, as with a simple gyroscope you're typically not putting net torque into the system. It just rotates with some wobble and no net change in angular momentum of the gyro.

The wheel example I put down is different: by holding the rope attached to one end of one peg, and letting the other peg free, there is a net torque caused by the offset in force between the upward force of the rope and the downward force of the Earth's gravity.

Most people, when seeing the setup before letting go, would think that the wheel will simply fall downward until the wheel is hanging straight from the peg. Or, if they have a bit of knowledge of angular momentum and gyroscopes, they might think that it will go downward more slowly than were it not spinning, or just hang there and spin. Neither is the case: the net torque enacted on the wheel causes it to rotate. In the example I set up earlier, if you look down on the wheel, the wheel will turn around the rope in a clockwise direction.

Due to friction and the twisting of the rope, it will slow down and eventually come to rest with the wheel horizontal and no longer spinning. But the primary motion is the turning of the wheel around the rope.

 

[Arwin]

All that your statements show is that we define time through observation.

Well, it was intended to show that we measure motion by motion, and call the unit of measurement time.

Remember that time isn't just about the fact that things move, but about the specific relationships between different motions.

Right.

The fact that we can predict the motions of physical systems to an extreme degree of accuracy using time attests to the fact that time, when combined with the other aspects of these theories, is an accurate description of reality.

Right. As long as we never forget what time means! I don't think we've disagreed about anything so far, and this has been, as so often, purely semantics based. I'm not a physicist, you're not a linguist. But I am an amateur 'know a little about everything' and I've often amused myself (and probably only myself ) trying to build bridges between the different fields, because (at least in my country) I'm a rare individual who's attended both the 'sciences' and 'arts' departments at university (and what a world of difference - what the one lacks in science, the other lacks in eloquence ).

 

[Bambers]

It's a standard trick included in the paper leaflet of most gyros, just set it going and rest it horizontally with one end on a finger or held by a loop of string.

Although the 'wobble' even if you put it upright is still the same precession effect.

I would presume the bike wheel has become quite common becuase it's large (so useful for lecture theaters) and easy to get a hold of. There used to be a small science based hands on museum nearby when I was younger which included the usual selection of bikewheel fun and games (eg, holding bike wheel on a freely rotating platform)

Another demonstration of the same thing is just set the gyro going and then try to turn it in your hand, not only will it resist being turned but you can feel it trying to turn in the relevant direction too.

and I do point to the part where I mentioned clockwise in my response.


Not sure any of that is really relevant though, my point was I don't believe that some things are more or less accessible to 'understanding' than others. I mean when you think about about it, why the heck does one object move when another hits it etc and then we start diving off into deep philisophical stuff like why do we exist? However, we do and things move when you hit them and we accept that and have made up some maths to model it, no one questions it hugely, it just is. Whats so different about conservation of angular momentum or SR or GR that makes it unacceptable?

 

[KimB]

Right. As long as we never forget what time means!

I'm not quite sure where this statement has application, however.

For the general public, I don't think it makes a whole lot of difference as the general public's idea of time isn't very specific anyway. They should just be aware that general terms like "time" are not exact representations of how they are used within science: within science terms are defined much more exactly.

Within science, all that is required is that scientists be aware of the limits of inductive reasoning, where we find that there is no statement about the nature of reality of which we can be absolutely certain. So even though we "understand" what we mean by time within a particular theory, meaning that we can use that definition to provide explicit experimental results, we accept that that theory is not necessarily exact. And, in fact, in most cases we know that the theory in question is only an approximation of the true behavior, as there are inconsistencies in the theory beyond where experiments have yet tested.

So I just don't quite see what the point of your criticism is.

 

[KimB]

Not sure any of that is really relevant though, my point was I don't believe that some things are more or less accessible to 'understanding' than others. I mean when you think about about it, why the heck does one object move when another hits it etc and then we start diving off into deep philisophical stuff like why do we exist? However, we do and things move when you hit them and we accept that and have made up some maths to model it, no one questions it hugely, it just is. Whats so different about conservation of angular momentum or SR or GR that makes it unacceptable?

Well, I don't think there's anything all that different, except perhaps the level of complexity. It only takes a tiny knowledge of torques and angular momentum to determine the behavior of the bicycle wheel in this case. But it's still the case that never having seen the setup before, even if the person is familiar with many other aspects of rotating bodies, most people seem to get the answer wrong unless they sit down and think about it.

This seems to often be the case with even Newtonian mechanics: much of the time, it seems natural, intuitive. But throw up a situation that's a little bit out of the ordinary, and suddenly it ceases making sense. General Relativity is like that, except for the fact that we can never see the more out-of-the-ordinary situations that we can describe on paper, such as behavior near a neutron star or black hole.

P.S. As for the gyroscope, I had been thinking of the instrument, not the education toy. I've never owned the toy version.

 

[purpledog]

Along the same axis...

I was always surprised that everything in the universe seems to rotate around itself. Then I had this fantastic intuition that actually, any object in "empty space" was:
- perhaps not moving at all
- or rotating around itself (the impulse could come from some meteorite, or many of them actually)

In a nutshell, if you remove any translation, the only thing left is a rotation around a constant axis. And yes, let's say I'm talking about Newtonian physics.

Nice, isn't it? Wait, there's a trick...